There is known, as one of output devices of computers and workstations, an image forming apparatus employing an ink-jet system in which ink is ejected from a print head to form an image on a recording medium such as a recording sheet. The image forming apparatus employing an ink-jet system comprises, for example, a print head having an ink ejection surface on which a plurality of ink ejection outlets each for ejecting ink are formed, a carriage on which the print head is mounted, said carriage reciprocating in a predetermined direction, and a recording paper conveying device for conveying recording papers in a direction (a recording sheet conveying direction) perpendicular to the predetermined direction.
In the event that an image is recorded on a recording sheet, the recording sheet in the course of conveyance by the recording sheet conveying device is temporarily stopped, and while the carriage is reciprocated in the predetermined direction, ink is ejected from the ink ejection outlets in accordance with an image signal having image information to form (print) a band of image on a portion located at an image formation area of the recording paper facing the ink ejection outlets. Thereafter, the recording paper is fed by a band of width and is stopped, and again while the carriage is reciprocated in the predetermined direction, ink is ejected from the ink ejection outlets in accordance with the image signal to form an image on a new portion located at the image formation area of the recording sheet. Such a performance is repeated and thereby forming an image on the recording sheet in its entirety.
According to such an image forming apparatus employing an ink-jet system, forming an image on a recording sheet through ejection of ink brings about a phenomenon (cockling) in which fibers of the recording sheet absorbing the ink partially expand and whereby unevenness is formed on the expanded portion and its periphery. It happens that unevenness due to the cockling is not only formed on the portion of the recording paper which have been subjected to the image formation through an ink adhesion, but also greatly grows up to the subsequent portion of the recording sheet which is ought to be subjected to the next image formation. In this case, the printing would be carried out on the top portion, the bottom portion and the slant portion of the unevenness. This brings about delicate deviation in a printing position and has a bad effect on an image formation thereby involving degradation of the image quality. Further, there is the possibility that a convex portion of the recording paper is in contact with the print head mounted on the carriage which reciprocates at high speed in the predetermined direction, and as a result portions, such as the ink ejection outlets and the like, which are precisely manufactured, are damaged. Furthermore, there is the possibility that paper powder and the like adheres to the ink ejection outlets and as a result the ink ejection becomes poor thereby involving degradation of the image quality.
As a technology for reducing the cockling, there has been proposed a technology in which a recording paper, that is random in directional properties of fibers, is used so that the recording paper extends in multi-directions, but not in a single direction by ink absorbed in the recording paper. According to this technology, however, the use of the recording sheet is restricted to only a specified recording paper, and the cost is expensive. Further, according to the above-mentioned technology, it is difficult to sufficiently reduce the cockling.
As another technology for reducing the cockling, there has been proposed a technology in which a recording sheet is wound onto a roller having a large diameter and is conveyed, so that a printing is carried out on the recording sheet wound onto the roller. In this case, however, in view of the size of the image forming apparatus in its entirety, there is a limit in the diameter of the roller. For this reason, distances between the recording sheet wound onto the roller and a plurality of ink ejection outlets are varied for each ink ejection outlet. In this case, of the formed image, portions, which are formed by ink ejected from the ink ejection outlets spaced apart from the recording sheet with relatively long intervals, undergo degradation of the image quality, and further it is difficult to expect a printing on a straight line basis. Furthermore, in this case, it is difficult to convey thick recording sheets, high rigidity of recording sheets, etc.
As still another technology for reducing the cockling, there has been proposed a technology referred to as a multi-scanning scheme in which a printing is performed on a recording sheet in such a manner that a carriage is reciprocated (scanned) over and over on the same plane (the same band) so that ink is ejected on the same plane of the recording sheet little by little on a divisional basis. According to this technology, however, the carriage is scanned on the same plane several number of times, and thus it takes a lot of time by the correspondence.
In view of the foregoing, it is an object of the present invention is to provide an image forming apparatus employing an ink-jet system, which is capable of preventing a damage for a print head due to the cockling and contributes to an improvement of an image quality.